Client devices such as personal computers (PCs) capable of network connection have become widespread. Along with the widespread use of the devices, the network population of the Internet is increasing. Various services using the Internet have recently been developed for the network users, and there are also provided entertainment services such as games.
One of the services for the network users is a multiuser online network game such as MMORPG (Massively Multiplayer Online Role-Playing Game). In the multiuser online network game, a user connects his/her client device in use to a server that provides the game, thereby doing match-up play or team play with another user who uses another client device connected to the server.
In a general multiuser online network game, each client device sends/receives data necessary for game rendering to/from the server. The client device executes rendering processing using the received data necessary for rendering and presents the generated game screen to a display device connected to the client device, thereby providing the game screen to the user. Information the user has input by operating an input interface is sent to the server and used for calculation processing in the server or transmitted to another client device connected to the server.
However, some network games that cause a client device to execute rendering processing require a user to use a PC having sufficient rendering performance or a dedicated game machine. For this reason, the number of users of a network game (one content) depends on the performance of the client device required by the content. A high-performance device is expensive, as a matter of course, and the number of users who can own the device is limited. That is, it is difficult to increase the number of users of a game that requires high rendering performance, for example, a game that provides beautiful graphics.
In recent years, however, there are also provided games playable by a user without depending on the processing capability such as rendering performance of a client device. In a game as described in International Publication No. 2009/138878, a server acquires the information of an operation caused in a client device and provides, to the client device, a game screen obtained by executing rendering processing using the information.
In the above-described game of International Publication No. 2009/138878, the game screen provided by the server to the client device is provided in the form of coded moving image data to reduce the information amount to be sent. A general moving image coding type such as an MPEG standard performs intra coding (intra-frame coding) without motion compensation for each block obtained by dividing one frame image or inter coding (inter-frame coding) with motion compensation by inter-frame prediction, although this changes depending on the adopted coding type. For each coding type, an object of high compression efficiency exists. In general, blocks (iblock and pblock) are generated by performing intra coding and inter coding for each block, and a block of high compression efficiency is included in coded data.
In intra coding, DCT, run-length coding, and the like are applied to an unprocessed block image of compression target, thereby compressing the image. On the other hand, in inter coding, the difference image between a block image of compression target and a reference image extracted from the preceding frame image in correspondence with the block is generated. Then, DCT, run-length coding, and the like are applied to the image to compress it. For this reason, inter coding includes processing of specifying, in the preceding frame image, a region having the highest correlation to the compression target block image. In the processing associated with detection of the region having the highest correlation, analysis is performed by calculating the similarity and distance to the compression target block image while moving the evaluation region in the preceding frame image. Hence, this analysis processing may take time.
Especially in, for example, a game that changes the rendered contents interactively based on user input, real-time rendering, that is, high-speed response to input is needed, and the time required for moving image coding processing is limited. In this case, it is necessary to quickly perform coding processing of each block and processing of determining whether to perform intra coding or inter coding. However, no detailed method of quickly and efficiently performing moving image coding of a rendered screen has been disclosed yet.